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Ahead of the flow

Hydraulic modeling is great for predicting potential system failures, but nothing beats data from the field. Building flow-monitoring services into the initial phases of a sewer project averts costly repairs down the road.

Smoke signalsA typical smoke test encompasses a small portion of a sewer line. Areas showing inflow and infiltration can pop up anywhere along the system. Source: V&A Consulting Engineers

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Smoke pours from a residential area drain illegally connected to a sanitary sewer system. When the yard becomes flooded, the stormwater is drained directly into the sanitary sewer. This is a direct inflow location. Photo: Oro Loma Sanitary District

FM technicians have years of experience observing flows within collection systems and they visit the collection system at multiple locations as part of their work. As a result, they often come across evidence or have strong suspicions regarding pipelines and basins contributing to I/I. If the consulting engineers and sanitary agencies agree to put a little extra into the budget up front, various I/I tracking techniques could be used to track down the source of the I/I right then and there. That way, small problems can be dealt with while they still are small, which saves the municipality a lot of time and effort in the long run.

Hydraulic modeling of wastewater collection systems occurs as part of the master planning process, and economic realities tend to dictate that FM is offered and regarded as a low-cost “commodity” service. Unfortunately, a waste-water collection system is neither uniform nor predictable, and a hydraulic model is only a theoretical construct. It can help predict how the system will behave under certain storm event conditions, and it can isolate certain basins and expose the system's limitations from a pipe-capacity point of view. But if what is needed is specific and actionable information, there's no substitute for “eyeball-verified” field data.

Public works departments should elevate FM from its customary status as a commodity service, and broaden its scope. This could be done in several ways.

Increase knowledge. Give FM technicians extra training and furnish them with information such as maps of the drainage basins, knowledge of problem areas, and prior sanitary sewer overflow locations. If team members are allowed to take action where warranted, they often can identify or eliminate potential problems. A little extra work can potentially accomplish what later would require many hours of effort to set up a separate study. The crews also could collect data for the district's permanent archive and confirm such seemingly obvious “details” as pipe diameters and locations, which often do not agree with existing documentation.

Include the engineer. In an ideal world, every FM crew might include an engineer, since the engineers—trained and interested in solving the municipality's problems from the master planning perspective—often can spot things that even a well-trained, experienced field technician might overlook. It's probably overoptimistic to suggest such a thing is possible, but the numbers do support the idea, since the hourly fee for an engineer is vanishingly small when measured against the value of the assets concerned.

Document the actual condition of the system. Being able to show that steps have been taken to identify and correct problems can keep public works or waste-water districts in good favor with the various regional, state, and federal oversight agencies.

Such an all-encompassing approach may not be possible in a medium or large city or district with its dense urban infrastructure. But even there it could be applied to one or two large basins that were already identified as problems in the previous master plan. And for small areas with limited resources, this approach can pinpoint problems far more quickly and cost-effectively than with other conventional methods.

— Krajewski is principal and manager of the sewer system evaluation services division of V&A Consulting Engineers, Oakland, Calif.